Humidity Control Calculator

Room Volume is a direct linear multiplier -- twice the cubic footage means twice the air to bring from Current Relative Humidity to Target Relative Humidity -- and it spans a much wider relative range here (10 to 5,000 cubic feet) than either humidity input (0-100%) or Room Temperature (50-110F). A small grow tent and a large flowering room at the exact same humidity gap need proportionally different equipment almost entirely because of that volume difference.

It's a direct comparison: if Current Relative Humidity is higher than Target Relative Humidity, the room has too much moisture and needs a Dehumidifier; if it's lower, the room needs a Humidifier to add moisture. There's no gray zone or minimum gap required -- even a one-point difference picks a direction, though the resulting Moisture Removal/Addition figure would be small. If the two percentages are exactly equal, Device Needed shows "None needed" instead, matching the 0 pints/day figure rather than defaulting to a misleading device recommendation.

Yes -- it's a standard psychrometric approximation for how much water vapor saturated air can hold at 70F and roughly sea-level pressure, consistent with ASHRAE psychrometric data (about 0.0158 lb of water per lb of dry air at saturation, translated through air density into grains per cubic foot). The calculator scales that reference point up or down for other temperatures using the Magnus formula for saturation vapor pressure -- the same approximation the National Weather Service publishes for its own vapor-pressure calculations -- evaluated as a ratio against its own value at 70F.

Because Est. Run Time and Est. Monthly Cost assume a device sized from one of two fixed capacity tiers -- 30 pints/day or 70 pints/day -- chosen by whether Moisture Removal/Addition crosses the 30-pint threshold. A figure just above 30 pints/day gets sized against the larger, higher-wattage tier, which can raise the cost estimate more than the underlying moisture figure changed.